In recent years, from the viewpoint of environmental concerns, fuel consumption, and the exhaust gas of automobiles, development of fuel injectors for automobiles using a laminated piezoelectric element has progressed.
The laminated piezoelectric element usually has a ceramic laminate that is formed by alternately laminating a piezoelectric layer of a piezoelectric material and inner electrode layers having electrical conductivity, and by having protection layers formed at both ends of the ceramic laminate in a lamination direction. An external electrode is connected to side surfaces of the ceramic laminate via an electrically conductive adhesive. A voltage is applied to between inner electrodes, thereby generating a displacement, in the piezoelectric layer, for driving.
When this laminated piezoelectric element is used for an injector, conventionally, various problems occur due to the injector being is used at a high temperature for a long period.
For example, an electrically conductive adhesive is coated on the side surface of the ceramic laminate to connect an external electrode to the ceramic laminate. However, there is a risk that the electrically conductive adhesive is peeled off from the side surface of the ceramic laminate due to the stress generated by a piezoelectric displacement, resulting in a failure in electric conduction. Particularly, protection layers that are formed at upper and lower ends of the ceramic laminate are applied with larger stress than that applied to other parts, due to the piezoelectric displacement. Therefore, the electrically conductive adhesive can be easily peeled off from these protection layers.
As a method of solving the above problems from the viewpoint of a material, use of a flexible material for the electrically conductive adhesive, that can cope with the piezoelectric displacement, has been proposed. However, because of the severe environment in which the electrically conductive adhesive is used, the usable materials are limited. Further, there is a limit to the performance of materials. Therefore, it is not effective to improve the materials.
On the other hand, configurations have not yet been improved. In Japanese Patent Unexamined Publication No. 9-270540, protection layers having an electrostriction material and an inner electrode laminated alternately are shown. However, no measure is taken to improve adhesive strength of the electrically conductive adhesive. Therefore, this proposal cannot solve the above problems.
The present invention has been made in the light of the above conventional problems. The invention provides a laminated piezoelectric element that suppresses the peeling off of the electrically conductive adhesive and has excellent durability and high reliability, and a method of manufacturing the laminated piezoelectric element.